Sales channels is where your product is sold. This can be on your ecommerce website, through Instagram, in an offline shop, through a reselling retailer, etc. It is where actually the conversion takes place. DTC is a blend where the site as a marketing channel is also the main sales channel.
Your marketing channels work on the general brand image and demand for your products, and can lead potential customers to one or many sales channels. For example, it is a common practice to list offline retailers on your website where you sell directly in case some cautious shoppers want to go and try the product in person before buying.